The absolute phase is defined as the shift between the maximum of the pulse envelope and the nearest maximum of the carrier wave of the electric field of a laser pulse. The absolute phase is frequently also referred to as carrier-envelope phase (CEP). This is, thus, a quantity that is required to describe the exact course of the electric field of laser pulses of any kind. CEP plays a central role, particularly in laser pulses whose pulse envelopes have a FWHM (full widths at half maximum) of only a few optical cycles, so-called few-cycle pulses or single-cycle pulses, since the electric field of these pulses has particularly large CEP-dependent asymmetries. CEP-dependencies are, however, also observed in experiments with multi-cycle pulses.
The generation of single-cycle pulses has been possible since 1997 (M. Nisoli, S. Stagira, S. De Silvestri, O. Svelto, S. Sartania, Z. Cheng, M. Lenzner, C. Spielmann, F. Krausz: A novel high energy pulse compression system: Generation of multigigawatt sub-5-fs pulses, Applied Physics B-Lasers And Optics, 1997, Vol. 65; 189-196). Since then, these laser pulses and their interactions with matter constitute a field of research of central interest and are extensively studied.
In 2001, effects of the absolute phase were directly detected in the photoionization of noble gases (G. G. Paulus, F. Grasbon, H. Walther, P. Villoresi, M. Nisoli, S. Stagira, E. Priori, S. De Silvestri: Absolute-phase phenomena in photoionization with few-cycle laser pulses, NATURE, 2001, Vol. 414, 182-184). In doing so, the CEP-dependent asymmetries of single-cycle pulses were observed in the form of spatially asymmetrically emitted photoelectrons. To this end, two oppositely arranged time-of-flight spectrometers (stereo time-of-flight spectrometers) were used.
Since then, other devices for CEP measurements have also been proposed and partially implemented. In those cases, it was usually necessary to average over effects induced by hundreds or thousands of laser pulses in order to determine the CEP of the laser pulses. Similarly, the majority of those methods would usually only work if the laser pulses are shorter than approximately two optical cycles (half width) (about 6 fs at 800 nm central wavelength of the laser). (T. M. Fortier, P. A. Roos, D. J. Jones, S. T. Cundiff, R. D. R. Bhat, J. E. Sipe: Carrier-Envelope Phase-Controlled Quantum Interference of Injected Photocurrents in Semiconductors, Phys. Rev. Letters, 2004, Vol. 92, No. 14; A. Apolonski, P. Dombi, G. G. Paulus, M. Kakehata, R. Holzwarth, Th. Udem, Ch. Lemell, K. Torizuka, J. Burgdörfer, T. W. Hänsch, F. Krausz: Observation of Light-Phase-Sensitive Photoemission from a Metal. Phys. Rev. Letters, 2004, Vol. 92, No. 7; M. Kreβ, T. Löffler, M. D. Thomson, R. Dörner, H. Gimpel, K. Zrost, T. Ergler, R. Moshammer, U. Morgner, J. Ullrich, H. G. Roskos: Determination of the carrier-envelope phase of few-cycle laser pulses with terahertz-emission spectroscopy, Nature Physics Let., 2006, Vol. 2, 327-331; C. A. Haworth, L. E. Chipperfield, J. S. Robinson, P. L. Knight, J. P. Marangos, J. W. G. Tisch: Half-cycle cutoffs in harmonic spectra and robust carrier-envelope phase retrieval, Nature Physics, 2007, Vol. 3, 52-57; G. G. Paulus, F. Lindner, H. Walther, A. Baltuska, E. Goulielmakis, M. Lezius, F. Krausz: Measurement of the phase of few-cycle laser pulses, Phys. Rev. Let., 2003, Vol. 91, Issue 25).
In 2009, it was for the first time possible to determine the CEP of single-cycle pulses with high accuracy using the above-mentioned stereo time-of-flight spectrometer arrangement in the single-shot mode (T. Wittmann, B. Horvath, W. Helml, M. G. Schatzel, X. Gu, A. L. Cavalieri, G. G. Paulus, R. Kienberger: Single-shot carrier-envelope phase measurement of few-cycle laser pulses, Nature Physics, 2009, Vol. 5; 357-362). This set-up, too, is only suitable for measuring the CEP of single-cycle pulses having a half-peak duration of less than 8 fs.
In 2010, a further method was demonstrated, which enabled the measurement of the CEP of ultrashort pulses having pulse durations of 38 fs in the single-shot mode (P. Tzallas, E. Skantzakis, and D. Charalambidis: Measuring the absolute carrier-envelope phase of many-cycle laser fields, PHYSICAL REVIEW A 82, 061401(R), 2010). There, a laser pulse with time-dependent polarization is used in interaction with a noble gas for the generation of radiation in the extreme ultraviolet range, so called high harmonics. The CEP of the ultrashort pulse can be determined from the exact measurement of the course of the spectral intensity of the extreme ultraviolet radiation. That method can, however, only be implemented at very high pulse energies (higher than 50 mJ), requiring comparatively expensive equipment. That method, moreover, involves high expenditures in the data transfer and calculation of the CEP such that the pulse repetition rate is limited. That method, thus, enables neither the determination of the CEP nor any influence on the CEP or a correlation of the CEP measurement with the measurement of other physical quantities in real time. In particular the need for very high pulse energies allows the use of that method as a basis for CEP control or a correlation of the CEP measurement with the measurement of other physical quantities in real time (CEP tagging) only for laser systems that provide low pulse repetition rates and pulse energies of several 10 mJ.
A device for the fast phase evaluation of single-cycle pulses was also already proposed (DE 10 2010 019 814.5), by which the determination of the CEP could be markedly improved based on the principle of the stereo time-of-flight spectrometer. That device allows for the single-shot determination of the CEP of single-cycle pulses in real time at repetition rates in the KHz range. At the same time, the equipment required for the CEP measurement is considerably reduced, and a high accuracy of the CEP measurement in the range below 200 mrad is achieved, while needing comparatively moderate pulse energies in the order of some 10 μJ. That device can thus serve as a basis for CEP control or a correlation of the CEP measurement (CEP tagging) with the measurement of other physical quantities in real time (A. M. Sayler, Tim Rathje, Walter Müller, Klaus Rühle, R. Kienberger, G. G. Paulus: Precise, real-time, every-single-shot, carrier-envelope phase measurement of ultrashort laser pulses, OPTICS LETTERS, Vol. 36, No. 1, 2011), although it is disadvantageous for the determination of the CEP that the output radius parameter R is reduced at the transition from single-cycle pulses to multi-cycle pulses, while the scatter Δr remains largely constant (A. M. Sayler, Tim Rathje, W. Müller, Ch. Kürbis, Klaus Rale, Gero Stibenz, and G. G. Paulus: Real-time pulse length measurement of few-cycle laser pulses using above-threshold ionization, Optics Express Vol. 19, Iss. 5, 2011, 4464-4471). Since the uncertainty of the CEP measurement ΔΦ approximately behaves like Δr/R (ΔΦ˜Δr/R), this leads to two essential problems for the CEP determination in real time and the single-shot mode:                1) The uncertainty of the CEP determination increases with the pulse duration, since the radius parameter R decreases at an increasing pulse duration, and ΔΦ˜Δr/R applies.        2) The CEP measurement will no longer be feasible, if Δr reaches the order of dimension of R, because the uncertainty of the CEP determination will become too large. The limit is typically at 8 fs (at a central wavelength of 800 nm).        
It is also known that laser pulses with time-dependently changing polarization directions are applied in the examination and optimization of the interaction of laser pulses with matter. The generation of laser pulses with time-dependently changing polarization directions takes place in a so-called polarization gating stage, which can be realized in various ways. (O. Tcherbakoff, E. Mével, D. Descamps, J. Plumridge, and E. Constant: Time-gated high-order harmonic generation, PHYSICAL REVIEW A 68, 2003, 043804; G. Sansone, E. Benedetti, F. Calegari, C. Vozzi, L. Avaldi, R. Flammini, L. Poletto, P. Villoresi, C. Altucci, R. Velotta, S. Stagira, S. De Silvestri, M. Nisoli: Isolated Single-Cycle Attosecond Pulses, Science 314, 2006, 443; P. Tzallas, E. Skantzakis, C. Kalpouzous, E. P. Benis, G. D. Tsakiris, D. Charalambidis: Generation of intense continuum extreme-ultraviolet radiation by many-cycle laser fields, Nature Physics, Vol. 3, 2007; P. B. Corkum, N. H. Burnett, M. Y. Ivanov: Subfemtosecond pulses, Optics Letters, Vol. 19, No. 22, 1994).
A special possibility is the use of different birefringent quartz plates of different thicknesses in combination with one or several Brewster windows (S. Gilbertson, Y. Wu, S. D. Khan, M. Chini, K. Zhao, X. Feng, and Z. Chang: Isolated attosecond pulse generation using multicycle pulses directly from a laser amplifier, PHYSICAL REVIEW A 81, 2010, 043810). So far, these techniques have been used to generate attosecond laser pulses. The use of these techniques for fast CE-phase evaluation is not known.